1464 A TEXTBOOK OF THEORETICAL BOTANY 



each is the characteristic Type. This system was adopted with sHght modifi- 

 cation by Johansen, who has enlarged it by subdividing each main Type 

 into a number of Variations, and has added a sixth Type. 



This scheme of classification is a practical or working classification, in 

 the sense that it is based upon comparison rather than arrived at through 

 the application of fundamental laws of development. Such laws are too little 

 understood yet to permit of their being used to contruct a workable classifi- 

 cation, though it has been attempted by Soueges in the treatment of his own 

 extensive observations. 



His laws are as follows, i . The Law of Origin. In any particular species 

 the sequences of cell formation may be established in such a manner that 

 the origin of the cells may be defined in exact terms by referring to the one 

 or to the other termini of the sequence. 2. The Law of Numbers. The 

 number of cells produced by different cell generations varies with the species 

 and depends on the rapidity of the segmentation in the cells of the same 

 generation. This law expresses the various rates of division of different 

 cells of the young embryo. 3. The Law of Disposition. The cells are con- 

 stituted by divisions in clearly determined directions and appear to occupy 

 positions in accordance with the part they must play in development. 4. 

 The Law of Destination. In any given species the cells of the young embryo 

 give rise to clearly determined parts and always to the same parts of the 

 embryonic body. 5. The Law of Parsimony. This is added by Johansen. 

 It follows the law of economy in reasoning known to philosophers as 

 "Occam's Razor" which runs thus: "Entities are not to be mukiplied 

 beyond necessity." It is a very pertinent limitation in such a subject. 



The classification proposed by Soueges is little more than an outline. 

 He recognizes four categories, i. Fundamental types or archetypes. 2. 

 Secondary or derivative types. 3. Superposed types. 4. Irregular types, which 

 are not definable under the laws. 



The archetypes are considered under two main divisions, first, those in 

 which the whole zygote conforms to the development laws and secondly 

 those in which it is only the apical cell of the two-celled embryo which con- 

 forms to the laws and the basal cell does not contribute to the development 

 of the embryo. These divisions are further divided into series and sub- 

 series. Six principal or megarchetypes are recognized in each division and 

 the series or sub-series are divided into groups in accordance with the 

 archetypes. 



This scheme has all the inflexibility of a logical deduction and it could 

 only be used by one who had the vast experience of Soueges himself, as it 

 presupposes a complete understanding of the operation of the laws. 



Soueges has also devised a scheme of embryological formulae for the 

 concise description of development. It is based upon a system of lettering 

 which denotes each of the significant cells or regions of the embryo and all 

 their derivatives. Thus, in the two-celled embryo the basal cell is called 

 cb and the apical cell ca. Although it is an essential tool for the specialist 

 we shall not attempt to use it here. 



